mailmap: replace strcpy with xstrdup

mailmap: replace strcpy with xstrdup

We want to make a copy of a string without any leading
whitespace. To do so, we allocate a buffer large enough to
hold the original, skip past the whitespace, then copy that.
It's much simpler to just allocate after we've skipped, in
which case we can just copy the remainder of the string,
leaving no question of whether "len" is large enough.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>